Coherent spin rotation-induced zero thermal expansion in MnCoSi-based spiral magnets

TL;DR

This study discovers a metallic MnCoSi-based spiral magnet exhibiting zero thermal expansion over a wide temperature range, achieved through coherent spin rotation that couples magnetic and lattice dynamics.

Contribution

It introduces a new metallic ZTE alloy and reveals a novel mechanism involving spin rotation-induced lattice changes for achieving ZTE.

Findings

Identified a cycloidal and helical spiral magnetic structure in MnCoSi alloys.

Demonstrated that spin rotation causes large negative thermal expansion along one axis.

Established a link between magnetic spin structures and zero thermal expansion behavior.

Abstract

Materials exhibiting zero thermal expansion (ZTE), namely, volume invariance during temperature change, can resist thermal shock and are highly desired in modern industries as high-precision components. However, pure ZTE materials are rare, especially those that are metallic. Here, we report the discovery of a pure metallic ZTE material: an orthorhombic Mn1-xNixCoSi spiral magnet. The introduction of Ni can efficiently enhance the ferromagnetic exchange interaction and construct the transition from a spiral magnetic state to a ferromagnetic-like state in MnCoSi-based alloys. Systematic in situ neutron powder diffraction revealed a new cycloidal spiral magnetic structure in bc plane at ground state which would transform to the helical spiral in the ab plane with increasing temperature. Combined with Lorentz transmission electron microscopy techniques, the cycloidal and helical spin order coherently rotated at varying periods along the c axis during the magnetic transition. This spin rotation drove the continuous movement of the coupled crystalline lattice and induced a large negative thermal expansion along the a axis, eventually leading to a wide-temperature ZTE effect. Our work not only introduces a new ZTE alloy but also presents a new mechanism by which to discover or design ZTE magnets.